The separation processes by means of membranes display remarkable advantages with respect to the energy expenses generated by the purification of mixtures either liquid or gaseous and conventional separation processes such as distillation, cryogenic separation, absorption, etc. The separation technologies based on membranes demand low operation costs and the different obtained products can be commercialized or reused due to their high degree of purity. The separation of gaseous mixtures by membranes is of great interest in many oil industry operations such as sweetening of natural gas, hydrogen separation in the ammonia purge currents, ethane separation plants, etc.
In the last two decades, the separation of gases by means of polymeric membranes has been focused on the use of vitreous polymers with aromatic structure and high vitreous transition temperature. In this type of structure, molecules with small kinetic diameters such as hydrogen and helium pass through faster, whereas voluminous molecules such as methane, nitrogen, ethane or propane pass through more slowly. The first industrial membranes used for the separation of gases were made of cellulose acetate. The main disadvantages featured by the membranes derived from cellulose are related to the limited selectivity/permeability ratio and the low thermal, mechanical and chemical stabilities. The global trend regarding the use of polymeric membranes is aiming at the application of high performance polymers such as polyimides, polyetherimides, polyamides, polybenzimidazoles, polytrimethylsilylpropine, polytriazole, among others.
Most patents dealing with the design of vitreous polymer membranes for the separation of gases date back from the 1980's-1990's as can be shown by the following information:
U.S. Pat. No. 4,230,463 issued in 1980 to Monsanto relates to the separation of a multicomponent mixture of gases using an asymmetric membrane (also known as anisotropic) made of commercial polysulfone, which included some chemical modifications such as the phosphonization, phosphorylation, sulphonation and inclusion of primary, secondary, tertiary and quaternary amines. The chemical structure of the commercial polysulfones are provided by Union Carbide (P-1700 and P-3500), 3M (Astrel 360 plastic), and ICI (polyether sulfone, polyarylene ether sulfone). Hollow fibers were fabricated by the Phase Inversion Method and later were covered with polymers that present a higher impairment to the flow of gases; among these coatings are found: polysiloxanes, polyurethanes, polyimines, polyamides, polyesters, cellulosic polymers, polypropylene glycol, polyethylene, polypropylene, polybutadiene, etc.
U.S. Pat. No. 4,474,858 issued in 1984 to UBE relates to the fabrication of porous aromatic polyimide membranes featuring the interstitial inclusion of a liquid for the separation of gases, specifically for the separation of hydrogen/carbon monoxide and nitrogen/oxygen. The chemical structure of the porous support is
where R represents a tetravalent aromatic radical and R1 represents a divalent aromatic radical. The radical R can have the formula:
R1 can have the formula:
where A represents a group among —O—, —S—, —CO—, —SO2—, —SO—, —CH2—, —C(CH3)2—; and where R2, R3 and R4 represent an atom of hydrogen, alkyl radicals of 1-3 carbon atoms or alkoxy radical with 1-3 carbon atoms and m is an interger between 1 and 4.
The main characteristics of the impregnating liquid are a boiling point of at least 180° C., and be incapable of dissolving the support, but capable of separating a gaseous mixture. In this case, naphthalenes either halogenated or alkylated can be used, in general, derived from naphthalene, aliphatic alcohols between 9 and 17 carbon atoms, aliphatic monocarboxylic acids between 9 and 17 carbon atoms and silicon liquid compounds, for example, polydimethylsiloxane, polymethyl phenyl siloxane and polytrifluoropropylmethyl siloxane.
U.S. Pat. No. 4,657,564 to Air Products and Chemicals, Inc. discloses fluorinated polymeric membranes for the gas separation process. The membrane prototypes were made of a polymer known as poly(trimethyl silyl propyne) with general formula:
where R1 is a linear or branched alkyl group C1-C4; R2 and R3 can be linear or branched alkyl groups C1-C6; R4 is an alkyl group, a linear aryl or branched alkyl group C1-C12; X is an alkyl group C1-C3 or
m≧100 y n=0 or 1. Such a membrane can be used efficiently in the separation of the following gas pairs: He/CH4, H2/CO, CO2/CH4, CO2/N2, and H2/N2.
U.S. Pat. No. 4,717,394 issued in 1988 to E.I. Du Pont de Nemours and Company relates to polyimide membranes with semiflexible chemical structures for the separation of gases. By controlling the rigidity of the polyimide molecule, the membranes can feature high permeation of gases and keep a suitable separation level of the gaseous mixture.
The family of polyimides have the general formula:
where: Ar is:
R can be:
or mixtures; Ar′ can be:
or mixtures; R′ can be:
or mixtures and R″ can be:
where n=1-4, X—X4 are alkyl groups C1-C6 or aromatics groups C6-C13; Z can be H or X—X4. The combination of the structures of the flexible amines with the rigid dianhydrides gives as a result chemical structures of semiflexible polymers, which promote the permeation of certain gases throughout the polymeric membrane. The membranes featured in this invention can be useful for the recovery of hydrocarbons in ammonia plants, the separation of CO/H2 in synthesis gas systems, the separation of either CO or CO2 from hydrocarbons and in the enrichment of either oxygen or nitrogen from air.
U.S. Pat. No. 4,964,887 to Nitto Denko Corporation relates to permeable membranes for the separation process of methane. The polyimide membrane has the formula:
where R1 can be a group of aliphatic, alicyclic and aromatic hydrocarbon or a divalent organic group. The membrane exhibits high selectivity and permeability to CO2 in the CO2/CH4 separation. In this multilayer membrane, both the polyimide support and elastomer film layer work as CO2 permeable materials. Typical examples feature either homo or copolymers of polypropylene, polyvinyl chloride, polybutadiene, polyisoprene, and polyisobutylene. The copolymers can contain functional groups such as acrylonitrile, (metha) acrylic esters, and (metha) acrylic acid. Intercross-linked silicon resins can also be used.
U.S. Pat. No. 5,074,891 issued in 1991 to Hoechst Celanese Corp. relates to the synthesis of membranes for the separation of gases. In this invention, polyimidic membranes are obtained by the Condensation Method by reacting fluorinated diamines such as 2,2′-bis(3-aminophenyl) hexafluoropropane, 2,2′-bis(4-aminophenyl) hexafluoropropane and 2-(3-aminophenyl)-2′-(4-aminophenyl) hexafluoropropane with aromatic dianhydrides such as the dianhydride of the 3,3′,4,4′ benzophenone tetracarboxylic acid. Membranes with high permeability and good separation factors are obtained.
U.S. Pat. No. 5,178,940 issued in 1993 to Nitto Denko K.K. relates to the formation of a composite membrane made of fluorinated polyimide type 6FDA with a film layer, and also of an asymmetric-no-composite membrane. The fluorinated polyimide structure is:
where R1 is a divalent aromatic, aliphatic or alicyclic hydrocarbon or a divalent organic group consisting of aromatic, aliphatic or alicyclic hydrocarbons linked to the other part of the divalent group. The thin film can be made of polyester, polyol, polyurethane, polyamide, epoxy resin, cellulose, etc. The permeation values and the separation factors are higher when a composite membrane is used instead of an asymmetric-no-composite membrane.
U.S. Pat. No. 5,334,697 to L'Air Liquide S.A. relates to a polyimide membrane for the separation of gases. In this invention, a separation membrane for at least one component of a gaseous mixture was obtained. The polyimide is obtained from xanthan dianhydrides 9,9-disubstituted and aromatic diamines. The dianhydride has the following structure:
where R and R′ can be —H, —CH3, —CF3, -phenyl, -substituted phenyl groups, alkyl groups or perfluoroalkyl C1-C16, preferably C1-C8. R and R′ can be similar or different. These polyimides present a suitable behavior for the separation of nitrogen and oxygen from air. This polyimide has the general formula:
where R and R′ are defined above; A is a diamine of the type:
mixtures; R″ can be:
or mixtures thereof; where R2 and R3 are alkyl or aryl groups; —X, —X1, —X2 and —X3 are alkyl groups; C1-C6 and the groups —Y, —Y1, —Y2 and —Y3 can be —X or —H.
U.S. Pat. No. 5,964,925 to Praxair Technologies, Inc. relates to gas separation membranes with sulfonated polyimides. The general formula of these compounds are:
where Ar1 and Ar2 are aromatic radicals. The aromatic rings contain radical groups of sulfonic acid (—SO3H, —SO3M or —SO2OR1), where M is an organic base, ammonium ion or alkali of the type K+, Na+, Li+, or a transition metal ion. R1 is an alkyl radical with less than C6 or an aryl radical, preferably methyl or ethyl. Are is an aromatic diamine represented by:
where R is a sulfonic group, Ar1 is represented among others by:

U.S. Pat. No. 6,896,717 issued in 2005 to Membrane Technology and Research, Inc. relates to a membrane that can be used for the separation of gases also containing hydrocarbon vapors (C3+). The base membrane incorporates a thin selective layer of a fluorinated polymer capable of protecting the membrane support from vapors and liquids of C3+ hydrocarbons. More specifically, it is used for the separation of hydrogen/methane, ethane or ethylene and carbon dioxide or hydrogen sulfide/methane, ethane or ethylene. The selective layer can be made of plyimide, polysulfone, cellulose acetate, among others. The membrane microporous support should present a low flow resistance and be preferably asymmetric. The dense layer free of defects is the one that carries out the separation and should be made of the same type of vitreous polymer as that of the support, for example, polysulfone, polyamide, polyimide, polyetherimide, polyvinylidene fluoride, etc. Such compounds should be preferably perfluorinated with a carbon:fluorine ratio of 1:1. The structure of the commercial polymer of Solvay Solexis, known commercially as Hyflon® is:
where x and y represent dioxol and tetrafluoroethylene, x+y=1. In some cases, the membranes can include agglutinant layers between the different constituents in order to coat the small defects on the support surface and avoid dragging the imperfections to the selective layer, or it also provides a layer of a highly permeable material that allows the connection of the pores in the support section. The sealing layer protects the thin permselective layer.
U.S. Patent Publication No. 2011/0290112 to UOP LLC relates to air separation using polyimide membranes. Such membranes can be fabricated as flat sheets or hollow fibres. These present an O2/N2 selectivity higher than 3 at 60° C. and a CO2/CH4 selectivity higher than 20 at 50° C. The general formula of these polyimides are:
where X can be:
or mixtures thereof. The physical structure of the membranes is asymmetric with a selective dense layer supported on a porous structure. Such membranes can be produced as flat sheets, disks, tubes, hollow fibers or thin films.
U.S. Patent Publication No. 2012/0323059 to UOP LLC relates to gas separation processes using polyimide membranes. A polyimide type is presented with a CO2 permeability of 50 Barrers and a CO2/CH4 selectivity of 15 at 50° C. Such a membrane features two groups susceptible of intercross-linking by UV radiation.
This polyimde has the following general formula:
where X1 can be:
or mixtures; X2 can be:
or mixtures; Y can be selected among others:
or mixtures.
U.S. Patent Publication No. 2013/0014643 to Membrane Technology and Research, Inc. relates to a conditioning process of fuel gas using vitreous polymer membranes. The process consists of the conditioning of natural gas that contains C3+ hydrocarbons and that can be used as feedstock for equipment that use fuel gas such as turbines and compressors. This process uses vitreous polymer membranes that permeate preferably methane above C2+ hydrocarbons to produce methane rich current. The membranes that can be used in this process comprise polyamides, polyimides, polysulfones, polyvinyl alcohol, polypropylene oxide, cellulose derivatives, polyvinylidene fluoride and polymers that contain fluorinated dioxole units, fluorinated dioxolones and fluorinated cyclic alkyl ethers. All these polymers permeate methane selectively over higher gaseous hydrocarbons. The selected fluorinated polymer is characterized by having a cyclic structure of at least 5 members, and such fluorinated rings are anchored to the main structure. The polymer should be perfluorinated with a carbon:fluorine ratio of 1:1, amorphous, present a Tg between 100 and 250° C. and not possess ionic groups that could give the membrane hydrophilic characteristics or affinity toward polar materials. That is to say that such a membrane should not feature a considerable swelling in polar solvents such as ethanol, isopropanol, butanol, acetone, acetic acid or water.
The first group of materials that can carry out such separation includes tetrafluoroethylene copolymers with the following structure:
where x and y represent dioxol and tetrafluoroethylene, x+y=1. Such materials are available under the name Hyflon® and are commercialized by Solvay Solexis, Inc.
The second type of polymeric materials for this application feature perfluorinated polymers of vinyl alkenyl ethers with members such as allyl or butenyl with the following structure:
These materials are commercialized under the name Cytop® and are produced by Asahi Glass Company.
The third group of selective materials for the same application is:
where x and y represent dioxol and tetrafluoroethylene, x+y=1 commercially known as Teflon® produced by Dupont.
Due to the fact that this class of polymers are vitreous and rigid, it is recommended that they be used as part of an asymmetric or composite structure. Preferably, a composite membrane containing a no-selective porous support and a film layer that gives it the required permeation properties should be used.